POSITION OF D0UBLE STARS. " 261 



•pparent change of place, which muft otherwife have hap- Obfervations and 



pened. In this cafe, no fingU.' niotion can be admitted to ex- J-pe^""^^' 



plain the phenomena of our double liar. But, if a real proper change of rcla- 



motion of t Bootis fliould hereafter be afcertained, the argu- ^j^^ fituation in 



..,,,,, ftars extremely 

 Itients we have ufed in the ca(e of 7 Leonis, will lead to the near each other, 



fame conclufion. 



Double Motions. 



(/) t and X cannot be the moving bodies ; and our former 

 argument (/) will apply to every double ftar v\ hatfoever. 



{g) O and X cannot be alone in motion; for, if no motion in 

 I can be perceived, it muft move in a fimilar manner with the 

 fun, and none of the three bodies will be at reft But, if its 

 proper motion fliall hereaf'er be found out, it muft either be 

 exactly the reverfe of the f )lar motion, and therefore only an 

 apparent one; or it will be more or lefs different. In the 

 latter cafe, all the three bodies muft be in motion ; in the 

 former, the exacl quantity of the folar motion will be difcO" 

 vered, and the relative parallax of many ftars may be had by 

 obfervation. 



(A) If O and « are the two bodies in motion, and if at the 

 fame time no motion in e can bo perceived, then the apparent 

 motion of x muft be intirely owing to the difl^erent efle6t of the 

 folar parallax on i and r; but (he effefl of the folar parallax on 

 X, can only be in a doedion contrary to the motion of the fun, 

 which, being north following the fmall ftar, whether it be 

 nearer or farther from us than e, muft have an apparent motion 

 towards the fouth preceding part of the heavens. But this is 

 diredly in oppofition to my obfervation of the motion of the 

 fmall ftar, which, thefe laft 23 years, has been direded tCf» 

 wards the north following. 



Motion of the three Bodies, 



(i) Let the motion of the fun be again towards x Herculis j 

 then, if no motion in e Bootis be perceivable, it muft movQ 

 exaftly like O. Highly improbable as it is, let it be admitted. 



Then, in addition to this extraordinary fuppofition, a third 

 motion is alfo required for x, which, aided by the (olar parallax, 

 is to carry it like wife within a quarter of a diameter of f, into 

 the fame place where, though unperceived, the large ftar has 

 hQQn carried b^ its own motion j that i^, in order to be appa- 

 rently 

 3 



